Background: This article describes the development of the Singapore Environmental Assessment Tool (SEAT), a culturally appropriate, usable, reliable, and valid assessment tool designed to evaluate the extent to which the built environment in Singaporean aged care facilities is able to support the provision of high levels of care to people living with dementia. Methods: A multistage sequential mixed methods approach was carried out involving 16 raters in Stage 1 and six raters in Stage 2 using a culturally adapted English version of the Environmental Audit Tool–High Care (EAT-HC) in eight nursing homes. The first stage captured qualitative data on cultural sensitivities and usability of the tool. The SEAT was improved based on the findings, and in the second stage, the usability and psychometric properties of the modified tool were again assessed. Usability was determined by data collected via the System Usability Scale and the Questions to Assess Barriers and Facilitators survey. Psychometric properties were examined by the calculation of percentage agreement, item-level interrater agreement was measured using Fleiss’s κ, and Cronbach’s α was used to measure the internal consistency of the subscale scores. Results: The culturally adapted SEAT was found to have an acceptable level of usability and moderate level of reliability among subscales. Conclusion: The study indicated that the tool is reliable and valid when completed by users with knowledge of dementia-enabling environments. For the tool to be used with confidence education in the application of principles of design to the design of environments for people living with dementia is recommended prior to its use.